1. Field of Invention
This invention relates to an image display system for use in a computerized tomograph, and more particularly to such a system which is capable of simultaneously displaying two images on the same screen, one image being representative of primary information and the other image being of additional separate information which can be selected as desired.
2. Description of Prior Art
In the art, there are known computerized tomographs for emitting a radiant ray, such as an X-ray beam, at a number of angles in a plane across a subject or patient, deriving a number of pieces of information (projection data) depending on the intensities of the radiant ray having passed through the patient, processing these pieces of information with a computer to determine distribution of radiation absorptivities of body sections in that plane for reconstructing a sectional image of the patient, and displaying the reconstructed image on a display screen such as a cathode ray tube.
Some computerized tomographs also include capability for displaying additional information together with the primary image displayed on the screen. FIG. 1 shows in block diagram form one example of a prior image display system having such an image display capability. Image data to be displayed on a cathode ray tube (hereinafter called "CRT") 50, are stored in an image memory 20. Image memory 20 has storage areas in a 1:1 correspondence to the pixels on the CRT screen. The image data stored in image memory 20 can be read out in the same sequence as that in which the CRT is scanned, in response to designation of pixel addresses by a pixel address generator 10. A display data processor 30 processes the image data as read out of image memory 20. The processed image brightness data are then combined by a composite signal generator 40 with a synchronizing signal issued for CRT display by a synchronizing signal generator (not shown) controlled by a central processing unit (CPU) 60. Composite signal generator 40 then delivers a composite video signal to CRT 50.
There are certain instances in which a secondary image is simultaneously displayed, by insetting the secondary image in the primary tomographic image on the CRT screen. Each time such an inset image is to be displayed, however, it has to be stored in a storage region 21 (see FIG. 2) to be extracted in primary image memory 20, which has addresses in a 1:1 correspondence to the pixel position addresses on the screen. This produces certain disadvantages, such as, for example:
1. When a secondary image is stored in the image memory, previous primary image information stored in the storage region, is destroyed, and hence such primary image data will be erased at the time the insetting mode is switched off.
2. Real time processing of image data from the image memory, such as window processing, level processing, blinking for flashing displayed image data pixels in a particular CT value zone, reverse video, and stressed image boundaries or edges, is affected equally throughout the screen. A display area to be processed and the remaining display area not being processed, cannot be observed simultaneously.
3. Where display data in the extracted storage region is of the same kind as that of data in the remaining storage region, they may be processed in the same manner. However, it would be more effective to differently process data of different kinds that are stored in the extracted region and in the remaining storage region. Since the primary and secondary images have been heretofore processed equally, either the primary image or the secondary image tends to become to the observer unequal in image quality with respect to the other, and it is difficult to compare the primary image with the secondary image.